The invention relates generally to correction of interpulse instability caused by radar amplifiers excited with dissimilar waveforms. In particular, the invention relates to characterizing the amplifier's transient behavior by cross-correlating a reference signal to an amplified signal, and a digital pre-distortion technique applied to the synthesized signal prior to amplification for correcting a transient input signal from the radar.
Modern radar systems are multifunction in design, supporting a diverse set of missions, each requiring a specialized set of waveforms. For modern multifunction radars, typical operating conditions include transmission of multiple diverse radar waveforms in close succession. Multifunction operation requires the use of several waveforms where duty cycle or pulse width varies from a previous waveform. Radar system engineers design waveforms with specific pulse width, duty cycle, and other requirements for optimal performance for each radar function. These waveforms are stored in a waveform library.
Multifunction capabilities emerge from radar's ability to rapidly change waveforms and arbitrarily steer the antenna beam. This means that at any given time, the radar can change waveform and search angle to perform a different function. The multifunction capabilities can cover a wide selection of functions such as air traffic control, volume surveillance, and dedicated tracking among others; all of these functions can potentially be executed simultaneously. The digital array radar (DAR) architecture, used in current and projected future radar system designs, offers the potential for increased dynamic range when compared to more conventional systems due to the use of a distributed receiver/exciter architecture. This enables higher clutter cancellation ratios, improves Moving Target Indication (MTI) and Pulse Doppler processing. Many processes taking place in a DAR depend on maintaining phase stability over a Coherent Processing Interval (CPI), and over extended periods of time (hours). Phase stability is necessary to achieve a high clutter cancellation ratio, which enhances the target detection capability in a cluttered environment.
Recent radar designs incorporate gallium nitride (GaN) monolithic microwave integrated circuit (MMIC) power amplifier (PA) technology into their final amplification stage. This is due to a high-power density that exceeds 4 W/mm, high efficiency, and high breakdown voltage. GaN PAs represent the state-of-the-art in terms of power density and efficiency, making them appropriate for radar applications. Thus, their properties at a system level need to be understood. Testing of solid state GaN PAs shows that, under multifunction conditions, the assumption of pulse-to-pulse stability lacks validity. These results are available from C. G. Tua: “Analysis of High Power RF Amplifier Electro-Thermal Memory Effects on Radar Performance”, 2012(a) and C. G. Tua: “Measurement technique to assess the effect of RF power amplifier memory effects on radar performance”, 2012(b). During multifunction operation the radar's power amplifier goes through thermal and electrical transients. These thermal and electrical transients are caused by electrical and electrothermal memory effects, and affect the complex gain of the power amplifier, creating instabilities that manifest as amplitude and phase transient at the amplifier's output signal. The transmitted waveforms are distorted, defeating the premise of identical pulse-to-pulse characteristic in a radar's coherent processing interval (CPI), also known as dwell time. Other potential instabilities have been reported, such as different pulse power due to staggered pulse repetition frequency (PRF), and long-time (hundreds of seconds) amplitude and phase drift due to device temperature changes.
Amplitude and phase (complex gain) transients have been observed to occur over time scales of several milliseconds on a CPI where the duty cycle or pulse width varies from the previous CPI. The time varying complex gain affects the effectiveness of stationary clutter rejection in Pulse Doppler radars, which presents a significant problem. For example, processing for moving target indication (MTI) is highly sensitive to instability in the system. Characterizing these amplitude and phase transients requires a set of sophisticated equipment capable of exciting a PA with multifunction radar waveforms and collecting data for processing. An instrumentation radar and test fixture in Tua, 2012(a), hereafter referred to as the PA Test Fixture, is used to study the observed transients.
To mitigate the amplitude and phase transients, conventional radars add pulses to the typical fill pulses, thereby attenuating the transients. These extra pulses are discarded before processing. This ensures the interpulse instability does not affect the radar's sensitivity. However, adding these pulses cost radar resources. RF energy is wasted reducing the overall efficiency of the system. Such time expended can't be employed to search or track legitimate targets. With the use of more dynamic multifunction waveforms, this fill pulse technique strains the scheduler with more radar required to mitigate the transients. There is no technique to properly mitigate the interpulse instability while minimizing the number of extra pulses needed to do so.